Dangerous mix-up's between regular insulin U-100 (100 units of insulin per mL of solution) and U-500 (500 units per mL) can occur. A mL is about 1/30th of an ounce and insulin vials usually contain 10 mL.

These errors could result in dangerously low blood sugars if people accidentally use U-500 instead of U-100 or dangerously high blood sugars if they’re supposed to be using U-500 and use U-100 instead.

Mistakes have usually occurred when doctors accidentally selected U-500 regular insulin from computer screens instead of U-100. For example, sometimes the two dosage forms appear one line apart on the screen, which makes it easy to select the wrong one. Also, depending on the screen size, the prescriber may see only the first few words of the product listing, so the drug concentration may not be visible. And finally, since the use of U-500 insulin is not common at present, prescribers may just assume that the only regular insulin that's available is U-100 and not even look for the concentration on the screen.

ISMP suggests that the use of U-500 insulin may be increasing due to the higher prevalence of diabetes, especially where patients are seriously overweight and may need higher insulin doses where they need U-500. The use of insulin pumps, and need for tight control of blood sugar in hospitalized patients are other reasons.

ISMP has asked the major suppliers of drug information systems to add the word "concentrated" on their selection screens, immediately following the drug name and preceding "U-500", which should help solve the problem. They have agreed to do this.

If you use insulin, be aware of the concentration and insulin type that you normally use. If you see U-500 on the label when you are supposed to be getting U-100, or if the opposite is true, make sure you question your pharmacist or doctor before taking it.